In high-speed communications systems, microwave electromagnetic energy, or simply microwaves, (i.e., electromagnetic energy waves with very short wavelengths ranging from a millimeter to 30 centimeters) are typically used as carrier signals for sending information from one place to another. Information carried by microwaves is transmitted, received, and processed by microwave circuits.
Microwave circuits require high frequency electrical isolation between circuit components and between the circuit and the world outside the microwave circuit. Traditionally, this isolation has been obtained by building the circuit on a shim, placing the circuit inside a metal cavity, and then covering the cavity with a metal plate. The metal cavity itself is typically formed by machining or casting metal plates and bolting, welding, or sealing them together using solder or an epoxy. This approach suffers from several limitations. First, machining is expensive. Casting is less expensive but is less accurate and, accordingly, metal cavities built using the casting method tend to have larger dimensions. This may result in parallel leakage paths around the microwave circuit component if the dimensions of the cavity are such to allow electromagnetic energy to propagate near the component's operating frequency. A further limitation in the traditional methods of building metal cavities is that the method of sealing the metal cover to the cavity has been to use conductive epoxy. The epoxy provides a good seal, but it has a high resistance, which increases the loss of resonant cavities and leakage from shielded cavities. As a result, the traditional isolation method using a shielded cavity has not yielded expected shielding isolation success rates. Finally, the traditional methods for shielding microwave circuit components requires significant assembly time. Accordingly, it would be desirable to have an inexpensive method for imbedding precisely-dimensioned low-loss shielded cavities in a microwave circuit package without additional parts or assembly.
Signals are generally propagated and guided throughout a microwave circuit using transmission lines and waveguides, both of which are known in the art. Transmission lines may take many forms, including but not limited to coaxial, coplanar, and microstrip transmission lines. Waveguides are generally hollow and provide many advantages over the other forms of transmission lines, including a simpler hollow pipe construction which does not require an inner conductor or associated supports, and their low-loss and low heat dissipative characteristics.
As known by those skilled in the art, electromagnetic signals travel entirely within a waveguide, reflecting off its inner surfaces according to the freespace wavelength .lambda. of the signal. In order for a signal to propagate inside the waveguide, the cross-sectional width of a waveguide must be greater than .lambda./2 of the dominant mode. The cross-sectional width .lambda..sub.c /2 of the waveguide determines what the cutoff frequency f.sub.c is, where .lambda..sub.c is the wavelength associated with the cutoff frequency f.sub.c. When the freespace wavelength .lambda. is long, it is low in frequency and approaches the .lambda..sub.c /2 dimension of the waveguide. When the cross-section width of the waveguide .lambda..sub.c /2 is less than .lambda..sub.c /2, the signal cannot propagate down the guide, and thus the waveguide acts as a high-pass filter in that it passes all frequencies above a critical or cutoff frequency f.sub.c.
Resonant cavities may be used to build microwave filters. A resonant cavity is a dielectric region completely surrounded by conducting walls. It is capable of storing energy and is analogous to the low-frequency LC resonant circuit. The resonant cavity is an essential part of most microwave circuits and systems. Every enclosed cavity with a highly conducting boundary can be excited in an infinite sequence of resonant modes. The frequencies at which resonance occurs depend upon the shape and size of the enclosed cavity. When a resonant cavity is placed along a transmission line, energy is coupled into the cavity at resonance and is reflected at other frequencies. A combination of resonant cavities in series with transmission line input and output couplers can be made to provide almost any kind of desired filter or response.
As with the shielding cavities described previously, waveguide structures and resonant cavities are traditionally formed by machining or casting metal parts, and then bolting, welding, soldering or using epoxy to fasten them together. This process is costly both in terms of the time and expense of forming each part and also in the assembly time required to put them together. Accordingly, it would be desirable to provide an inexpensive method for forming imbedded waveguide structures precise dimensions in a microwave circuit package which does not require an expensive fabrication and assembly of a lot of parts.